Dosage forms for delivering a beneficial agent, mainly a beneficial drug, to environments of use are known to the prior art in U.S. Pat. No. 3,845,770 issued to Felix Theeuwes and Takeru Higuchi, and in U.S. Pat. No. 3,916,899 issued to the same patentees. The dosage forms disclosed in the patents comprises a wall that surrounds an internal compartment containing the beneficial agent. The wall is permeable to the passage of an external fluid and substantially impermeable to the passage of beneficial agent. There is at least one passageway through the wall for delivering the beneficial agent from the dosage form. These dosage forms release the agent by fluid being imbibed through the wall into the compartment at a rate determined by the permeability of the wall and the osmotic pressure gradient across the wall to produce an aqueous solution containing agent that is dispensed through the passageway from the dosage form. These dosage forms are extraordinarily effective for delivering an agent that is soluble in fluid imbibed into the dosage form and exhibits an osmotic pressure gradient across the wall against the external fluid.
A pioneer advancement in dosage forms was presented to the delivery arts by Richard Cortese and Felix Theeuwes in U.S. Pat. No. 4,327,725. The invention in this patent enhances the delivery kinetics of the dosage form, for delivering agents with various degrees of solubility in aqueous fluids that are difficult to deliver, by manufacturing the dosage form with a hydrogel. The hydrogel in presence of fluid that enters the dosage form, swells and moves from a rested state to an expanded state. The increase in volume of the hydrogel acts as a driving force that is applied against the beneficial agent thereby urging the beneficial agent through the passageway from the dosage form.
The dosage form operates successfully for its intended use, and it can delivery numerous difficult to deliver agents. Its use, however, can be limited for agents that exhibit a high degree of solubility in an aqueous medium that enters the dosage form. That is, these agents can migrate into the fluid-expanding hydrogel become entrapped therein and, consequently, they are not available for delivery from the dosage form. It will be appreciated by those versed in the delivery arts, that if a dosage form is provided comprising means for substantially preventing the migration and the entrapment of the beneficial agent, such a dosage form would have a positive practical value and it would also represent both an unobvious improvement and advancement in the dispensing arts.